Internal combustion engines are heat engines that transform the energy from a chemical reaction into mechanical energy, and use their own combustion gases as a working fluid. They basically comprise two main parts: one or more cylinder heads and the engine block.
The valves of an internal combustion engine are housed in the cylinder head(s) and are intended to allow or block the entry or exit of gases into/from the engine cylinders.
Internal combustion engines contain intake valves which control the entry of gas mixture into the engine cylinder, and exhaust valves which allow the exit of the gases after explosion.
Given the working conditions demanded of such engine components during the operation of internal combustion engines, failure modes are observed in valves, especially in exhaust valves. In particular, one of these failure modes is corrosion, the result of which is extremely damaging to the combustion engine, leading to the loss of its performance and, eventually, engine shutdown with the required maintenance thereof.
Until now, among the most common solutions for lining engine valves of the prior art has been nitriding, which gives a negative performance on fatigue strength, for example. Another example is titanium valves, which are used for racing engines, but have a very high cost and low wear resistance, due to their surface being coated with titanium nitride (TiN) or titanium oxide (TiO) to compensate for low wear resistance.
There are also some additional solutions for engine valves that make use of commercially known alloys like Nimonic or Nireva, but the cost of these materials is not worth the properties offered for most situations.
Although there are various attempts to try to minimize the wear to which the valves are subject, the prior art solutions do not provide an internal combustion engine valve which manages, at the same time, to provide superior performance in all matters of durability.
Thus, one of the phenomena that most affects the durability of prior art valves arises from intergranular corrosion (IGC).
The phenomenon may be described as corrosion that begins at the grain edge. Due to exposure to high temperature, the chromium of the alloy migrates to the grain edge, i.e. the formation occurs of a precipitate of chromium in the grain boundary area. As a result, the loss of chromium as an element of the alloy, essential to corrosion resistance, leads to the dissolution of the grain boundaries and adjacent areas.
Another mechanism of wear that generally occurs in prior art valves is known as Hot Gas Corrosion. Hot gas corrosion to which valves are subject, is, generally, a uniform mechanism of corrosion associated, in most cases, with the hot gases to which exhaust valves are subjected. It is generally related to oxidation, but it may also occur through the attack of molten salts, such as sulphidation (sulphate salts formed by fuel and lubricating fluids).
It should be noted that the valves most affected by hot gas corrosion are exhaust valves, since it is these that receive heat resulting from the explosion in the combustion chamber. More particularly, it is the neck area of the valve that suffers greater wear due to corrosion, since, for reasons of valve geometry, it is this area that is most exposed to hot gases.
However, it should be stressed that this neck area of the valve suffers even more corrosion than other parts due to the manufacturing process of same, given that this is the portion of the valve subjected to the greatest plastic deformations and the need for subsequent heat treatment. Accordingly, the metallic chromium that was initially solubilized in the valve structure with a quantity of at least 17%, can no longer ensure this value and, as a result, is unable to offer the necessary corrosion resistance referred to above.
A third common phenomenon that attacks valves is illustrated in FIG. 11. In this case, a fault in the valve that prevents the rotary movement thereof may result in a small opening that allows the passage of gases from combustion. These gases, in their turn, since they have a high temperature and are corrosive, lead to corrosion in the area of the valve seat.
Such an occurrence prevents the proper seal that the valve has to provide. In some cases localized melting can occur, accelerating the phenomenon of corrosion until the valve fails. This occurs because the constant passage of hot gases drastically increases the temperature in a localized and concentrated area (see arrows in FIG. 11), making it impossible for the valve to provide correct engine operation. It should be noted also that this phenomenon has particular implications when the valve presents sealing problems.
For all the above reasons, until now a valve has not been developed for use in an internal combustion engine, in which at least one of the valve areas (such as the neck) exposed to a corrosive environment has been provided with at least 17% solubilized metallic chromium, providing the valve with high durability, simplified manufacture and resistance to corrosion and fracture.